1. Field of the Invention.
The present invention relates generally to a tubing conveyed perforating gun of the type used to perforate a cased well bore for the production of well bore fluids and, specifically, to a shock absorbing sub which automatically disconnects the perforating gun from the tubing string when the gun has been fired.
As oil and gas well bores are drilled, the integrity of the bore hole is preserved by cementing a casing or liner in place in the bore hole. The casing or liner is a metal, cylindrical conduit which must be punctured or perforated over the desired production interval in order to produce well bore fluids once drilling is complete. A perforating gun which utilizes some form of fired projectile and an explosive charge is used to perforate the casing or liner to begin production from the well.
Prior perforating gun techniques have either utilized tools which were run on a wireline or cable or have utilized tubing conveyed devices which were run on a tubing string to the desired depth in the well bore. Tubing conveyed devices have certain advantages over wireline methods, for example, in allowing safe, immediate release of formation pressure at maximum pressure differentials into the tubing string. With tubing conveyed perforating systems, the tubing can be run into position, a packer set to seal off a well bore, and the surface wellhead equipment can be installed. The packer setting can be checked by circulating fluid under pressure through the well annulus or through the well tubing string. Once the top side work is completed and tested for safety, the perforating gun can be fired to bring in the well. Since all surface work is completed before the perforating gun is fired, operating safety is enchanced.
Once the perforating gun has been fired and the casing is perforated, it is desirable to release the perforating gun portion of the device from the remainder of the tubing string leading to the well surface. This provides a greater flow area through the tubing string for production of well bore fluids and also allows tools and other devices to be run through the interior bore of the tubing string without contacting the perforating gun apparatus.
The present invention has as its object the provision of a disconnect sub for incorporation in a well tubing string between the perforating gun and the remainder of the string leading to the well surface, the sub service to automatically disconnect the perforating gun from the remainder of the tubing string once the gun has fired.
Another object of the present invention is the provision of such a disconnect sub which absorbs the impact of upward movement of the tubing string, usually thrust against the packer, caused by firing of the perforating gun.